This application relates to passively mode-locked lasers.
Ultra short optical pulses can be used in various applications including optical information processing and data communication, optical probing with high temporal resolution, laser surgery, and material processing. Mode-locked lasers can be used to generate such ultra short optical pulses. Such a mode-locked laser has a laser resonator to support multiple longitudinal modes that oscillate simultaneously. A mode-locking mechanism is implemented in the resonator to synchronize the phases of different modes and thus to lock these modes. These phase-locked modes constructively add to one another to produce a short laser pulse. Passive mode locking uses at least one nonlinear optical element inside the resonator to produce an intensity-dependent response to an optical pulse so that the pulse width of the optical pulse exiting the nonlinear element is reduced. Commonly used passive mode locking techniques include saturable absorbers, nonlinear fiber-loop mirrors (e.g., figure eight fiber lasers), and intensity-dependent nonlinear polarization rotation. The laser resonator may use various cavity configurations such as linear, ring, and figure-eight geometries.
Passively mode-locked lasers may be configured as fiber lasers where one or more fiber segments are used to transport or guide light in the laser resonator. Fiber lasers have been developed as a new generation of compact, inexpensive and robust light sources. A passively mode-locked fiber laser can be implemented by using an optically-pumped resonator with a doped-fiber as the gain medium. Many different dopants can be used to achieve laser oscillations at different wavelengths. Atomic transitions in rare-earth ions can be used to produce lasers from visible wavelengths to far infrared wavelengths (e.g., 0.45 μm-3.5 μm). Er-doped fiber lasers for producing optical pulses at 1.55 μm are particularly useful for optical fiber communication applications.
The laser pulses from a passively mode-locked laser are periodic in time and the pulse repetition period between any two sequential laser pulses should ideally be a constant. The pulse repetition rate or the pulse repetition frequency is the inverse of the pulse repetition period and is dictated by the optical path length of the laser resonator. In actual fiber and other passively mode-locked lasers, the optical path length of the laser resonator may fluctuate and drift with time due to various factors such as changes in temperature, vibrations and other perturbations. Such fluctuations and drifts in the optical path length of the laser resonator can cause the timing of the laser pulses to vary and such timing variation is commonly referred as timing jitter due to its random nature. Such timing jitter produces phase noise in the laser pulses produced by the passively mode-locked laser and can limit the performance of the laser in various applications. For this and other reasons, the timing jitter is undesirable and should be eliminated or minimized below an acceptable level in various applications.